A switching mode power supply typically consists of a pulse width modulation (PWM) control integration circuit and a MOSFET. With the development and innovation of power electronic technology, switching mode power supply technology is also constantly improving. At present, switching mode power supplies are widely used in almost all electronic devices due to its small size, light weight, and high efficiency, and have become an indispensable power supply for the rapid development of the electronic information industry today.
In switching mode power supplies, wire-wound fuse resistor is usually used by people as overcurrent protection for switching mode power supply products. Although the wire-wound resistor is also capable of cutting off the overcurrent by fusing, since its resistance wire is made of high-melting-point alloy, only when the power is over ten or more times of the rated power of the resistor, the alloy wire of the wire-wound resistor would be overheated and therefore fused in a short time, under such circumstance, the fuse wire function against fault current of the wire-wound fuse resistor is reflected. However, in practical applications, when the load is in abnormal condition, the current flowing through the wire-wound fuse resistor is often below the fusing current, such that the fusing function of the wire-wound resistor does not work while the surface temperature of the wire-wound resistor reaches 300° C.˜500° C. or even higher, which makes the devices such as chargers etc. unsafe, and raises a risk of fire. To solve this problem, the wire-wound resistor is externally connected to a thermal fuse in series and placed together with the thermal fuse inside a ceramic box. When the heat of the wire-wound resistor reaches the rated temperature of the thermal fuse, the thermal fuse gets cut-off, thereby cutting off the circuit. However, the method of externally connecting the thermal fuse in series beside the wire-wound resistor must occupy two areas on the PCB and requires four pads. Moreover, the heat transfer is not reliable enough, and the reliability of cutting-off according to temperature is poor.
In a currently used thermal fuse resistor, the thermal fuse is externally connected to the wire-wound resistor, and a lead wire of the thermal fuse is connected to a lead wire of the wire-wound resistor by spot-welding to form a series-connected structure. The thermal fuse resistor is relatively smaller in size and has a better over-current and over-temperature protection, but cannot realize the axial taping function and cannot meet the demand of automatic plug-in at the client end.
In another thermal fuse resistor existing in the current market, the thermal fuse is configured inside the wire-wound resistor, a lead wire of the thermal fuse is connected to an end cap of the wire-wound resistor, so that the thermal fuse and the wire-wound resistor form a series-connected structure, and the other lead wire of the thermal fuse and the other lead wire of the wire-wound resistor are led out in the same direction. This type of thermal fuse resistor has small size and good over-current and over-temperature protection function, but cannot realize the axial taping function and meet the demand of automatic plug-in at the client end.